Method and apparatus for recording high range resolution radar data

ABSTRACT

A method and apparatus for recording analog high-range resolution radar target signatures from single airborne targets encompassed within a moving range window on digital magnetic tape formatted for direct analysis by general purpose computers. Target echoes in the form of one-dimensional range profiles of radar backscatter along target length from each of a successive number of transmitted pulses are sampled and recorded at incrementally increasing range positions within a range window until the window has been completely sampled. The apparatus samples each signature at each occurrence of a sampling pulse from a timing unit controlled by a range tracker and holds the obtained sample until the next pulse is provided. Each time a complete range window is sampled, the moving sample-point sampling sequence is repeated and another signature is reconstructed. The sampled output is time-multiplexed with range, azimuth, and elevation data and recorded in digital form on digital magnetic tape which can be utilized in general purpose computers to analyze signature variation with respect to target aspect variation.

United States Patent Mott et al.

[ 1 May 9, 1972 of San Diego; Albert T. Roome, Escondido, all of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: July 27, 1970 [21] Appl.N0.: 58,384

Primary E.\'aminerT. H. Tubbesing Attorney-R. S. Sciascia, George JRubens and .l. W. McLaren [57] ABSTRACT A method and apparatus forrecording analog high-range resolution radar target signatures fromsingle airborne targets encompassed within a moving range window ondigital magnetic tape formatted for direct analysis by general purposecomputers. Target echoes in the form of one-dimensional range profilesof radar backscatter along target length from each of a successivenumber of transmitted pulses are sampled and recorded at incrementallyincreasing range positions within a range window until the window hasbeen completely [52] U.S. Cl. ..343/5 SA, 343/5 PC, 343/l7.2 PC sampledThe apparatus Samples each Signature at each occup ..h ..G01S 9/02 renceof a p g pulse from a timing unit controlled by a 1 0 "343/5 5 PC rangetracker and holds the obtained sample until the next pulse is provided.Each time a complete range window is sam- [56] References pled, themoving sample-point sampling sequence is repeated UNITED STATES PATENTSand another signature is reconstructed. The sampled output istlme-multlplexed with range, azimuth, and elevatIon data and 2,905,8949/ l 959 Rudmann ..343/5 SA X recorded in digital form on digitalmagnetic tape which can be 3,355,734 I 1/1967 Albanese... ....343/5 SA Xutilized in general purpose computers to analyze signature variationwith respect to target aspect variation.

3 Claims, 2 Drawing Figures INPUT WAVEFORM w IO M sg m r z u RECEIVERDETECTOR z z f-iv SAMPLE PULSES sl4 IL RANGE TIMING TRACKER TIMINGPULSES r |6 8 L 2 2 -I RANGE DATA MULTIPLEXING A/D MAGNETIC AZIMUTH DATAuNIr CONVERTER TAPE ELEVATION DATA -9 RECORDER PATENTEIIIIII 9 I9723,662,388

INPUT WAVEFORM w IO M SgWLLUETD SAMPLING REcEIvER DETECTOR UNIT SAMPLEPULSES sl4 l2 RANGE TIMING TRACKER TIMING PULSES T UNIT RANGE DATAMULTIPLEXING A/D MAGNETIC AzIIvIuTI-I DATA UNIT CONVERTER TAPE ELEVATIONoATA-- RECORDER I. I T-TRANSMISSION 1 I *ll T2 I TRANSMISSION 2 s 1 pf\/ I x I TRANSMISSION 3 s 1 M 4 I TRANsMIssIoN 4 S l O WK T l:TRANSMISSION K FIG. 'I

TIME FROM RADAR TRANSMISSION(RANGE) IN VENTORS ALBERT T. ROOME U QWMA/ATTOR N EYS METHOD AND APPARATUS FOR RECORDING HIGH RANGE RESOLUTIONRADAR DATA STATEMENT OF GOVERNMENT INTEREST The invention describedherein may be manufactured and used by or for the Government of theUnited States of America for governmental purposes without the paymentof any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Investigations by the Navy have demonstratedthe usefulness of high-range resolution radar for classification ofcertain radar targets. Results from the above investigations haveindicated the unique character of signatures of certain classes oftargets over a certain range of aspect variation by utilizing high-rangeresolution radar systems of the type described in An ExperimentalHigh-Resolution Radar for Target Signature Measurements," IEEETransactions on Aerospace and Electronic Systems, Vol. AES-3, No. 6, pp.249-256, Nov. 1967, by .l. N. Maynard and B. F. Summers. In the firstphase of the investigations using the l-foot range resolution radarsystem described in the above publication, target signatures wererecorded on photographic film using conventional photographictechniques. Although the techniques utilized were relatively simple toimplement, several inherent disadvantages limited the usefulness ofphotographic techniques. For example, slow data acquisition ratesprecluded study and analysis of target signature variation with respectto target aspect. Also conversion of photographic data into a formcompatible with general purpose computers was tedious and veryinaccurate and unreliable. Finally, it was not possible to usephotographic data for real-time signal processing hardware development.

SUMMARY OF THE INVENTION A method and apparatus for recording analoghigh-range resolution radar target signatures on digital magnetic tapeformatted for direct analysis on general purpose digital computers isdisclosed. A high-range resolution radar transmits a linear FM (chirp)signal having a range resolution in the order of l foot that makespossible the resolution of echo returns from individual scatterers alongthe target length so that the resulting target signatures are uniquelyrelated to the geometric configuration of the individual scatterers orscatter centers on the target. Each of a selectively predeterminednumber of signatures obtained from an equal number of successive radartransmissions is sampled in a moving sample-point mode by means of widebandwidth sampling apparatus. The apparatus samples each signature ateach occurrence of a sampling pulse and holds the sample until the nextpulse is provided. Each time a complete range window is sampled, thesampling sequence is repeated such that successive signatures aresampled and reconstructed. The sampling interval is determined by thefidelity required in the signature reconstruction and length of therange window is determined by the length of the largest target expected.The sampled output and range, azimuth, and elevation data aretime-multiplexed to an A/D converter that produces a digital output. Thedigital output is recorded on digital magnetic tape that can be readinto a general purpose computer that utilizes the range, azimuth, andelevation data to compute the aspect angle of the target both in azimuthand elevation thereby allowing the study of target signature variationwith respect to target aspect.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide a technique and apparatus for recording high-range resolutionradar target signature data in a digital form that can be utilized withgeneral purpose computers.

It is another object of the present invention to provide a dataconversion technique and apparatus whereby analog high-range resolutiondata can be studied for real-time variation with respect to targetaspect by means of general purpose digital computers.

It is another object of the present invention to provide means forobtaining high-range resolution radar cross-section records of radartargets.

It is another object of this invention to provide means for determininglocations of high radar reflectivity on vehicles such as aircraft orground vehicles.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified schematic blockdiagram of a highrange resolution radar target signature sampling andrecording system embodying the inventive concept disclosed herein.

FIG. 2 is a simplified graphical representation of the movingsample-point sampling technique disclosed herein as applied to a numberof typical successive target signatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT As discussed previously, thehigh-resolution radar used to obtain target signatures of the type to bediscussed hereinafter essentially comprises a radar of the typedescribed by Maynard and Summers in the previously mentionedpublication. The publication describes an S-band radar which transmits alinear FM (chirp) signal having approximately a 600- Mhz bandwidth and a0.3 microsecond duration.

The resulting time-bandwidth product is 180 and the range resolution isin the order of 1 foot. Other system parameters are described in thepublication, however, for the purpose of describing the presentinventive concept, the most significant parameter is the 1-foot rangeresolution.

This resolution makes possible the resolution of echo returns fromindividual scatterers on radar targets so that target signatures areuniquely related to the geometric configuration of the individualscatterers.

FIG. 1 is a simplified schematic block diagram of a radar targetsignature sampling and recording system embodying the present inventiveconcept. In the figure, detected high-range resolution radar echoes(returns) from a typical target being illuminated by high-rangeresolution radar pulses comprise input waveforms W to a high bandwidthsampling unit 10. The unit 10 essentially comprises a samplingoscilloscope which is used for both sampling and visual displaypurposes.

A range tracker I2 is connected at its output to the input of a timingunit 14. Moving airborne targets are range-tracked by setting andmaintaining a range window over a target. The window should encompassthe target length, e.g., for a 60-foot target the window width must beat least ns. A pulse with a variable time delay from the transmit signalestablishes the start of the range window.

The timing unit 14 is connected at its output to an input of thesampling unit 10. The timing unit 14 provides a selectivelypredetermined sampling sequence, and the range tracker generates timingpulses at the radar prf which occur at a selectively predetermined fixedtime prior to the occurrence of a target signature at the detectoroutput.

It can be appreciated that the timing pulses must therefore be movedcontinuously in order to maintain the above relationship which isgraphically illustrated in FIG. 2 wherein successive signature waveformswith increasing range for a moving target are shown. Signature waveformsW W W W W,,. are produced by the target in response to the successivehigh-resolution radar transmissions l, 2, 3, 4, k, respectively.

The range tracker 12 produces the timing pulses T T T, shown in FIG. 2which are coupled to the timing unit 14. The timing unit 14 functions inresponse to the timing pulses to initiate a moving sample-point modeconsisting of a selectively predetermined sequence of sample pulses S SS It can be seen that the successive sample pulses are delayed byincreasing multiples of a selectively predetermined fixed delay D. Forexample, the sample pulse S is delayed an amount equal to D after theoccurrence of its corresponding timing pulse T and in a similar mannersample pulse S is delayed an amount equal to kD after the occurrence ofits corresponding timing pulse T The sampling unit 10, which essentiallycomprises a sampling oscilloscope, samples the input waveform at theoccurrence of each sampling pulse and holds the obtained sample untilthe occurrence of the next sampling pulse. As previously mentioned theunit comprises a wide bandwidth sampling unit. For example, for theradar system parameters described previously, the unit 10 would have abandwidth in the order of 1 GHz.

When the delay interval reaches kD as shown in FIG. 2 the sequence isrepeated and another signature is reconstructed. The width D of thesampling or delay interval is determined by the fidelity desired in thesignature reconstruction and the interval length kD, or the length ofthe range window, is determined by the length of the largest targetexpected to be observed.

The reconstructed signatures, i.e., sampled outputs, are recorded onmagnetic tape by means of the recording apparatus of FIG. 1 wherein thesampled outputs, and range, azimuth, and elevation data are coupled to amultiplexing unit 16 which functions to time-multiplex the severalinputs to a conventional A/D converter 18.

The output of the A/D converter 18 is recorded in digital form by aconventional tape recorder 20. The recorded magnetic tapes can be readinto general purpose computers which utilize the range, azimuth, andelevation data to compute the aspect angle of the target both in azimuthand elevation. Target velocity can also be computed by the computers.

A number of tape characters corresponding to target signatures can benormalized to have a selected peak value that can be recorded on acomposite tape that contains discrete sets of signatures obtained fromknown targets. These composite tapes can facilitate computer analysis ofthe signatures of a target under surveillance by providing a library ofsignatures from various targets on a single tape.

Obviously, many modifications of the invention are possible within thescope of the disclosed inventive concept. For example, instead of usinga moving-sample-point mode to record target signatures as previouslydescribed, a stationary samplepoint, high-sampling rate mode can be usedto record modulated backscatter from rotating structures on targets suchas propellers.

What is claimed is:

1. The method of converting analog high-range resolution radar data fromremote targets into digital data compatible with general purpose digitalcomputers comprising the steps of:

illuminating a target under investigation with linear FM pulses ofmicrowave energy having a range resolution of approximately 1 foot;

receiving successive reflected one-dimensional range profiles ofmicrowave energy backscatter along the length of said targets; saidrange profiles being unique for each different remote target inaccordance with the geometric configuration of the scatter centers oneach of said different remote target;

detecting each of said successive reflected range profiles from saidtarget under investigation; sampling each of the detected successiverange profiles at a selectively predetermined rate and at incrementallyincreasing range positions corresponding to the target length to therebyproduce an analog digital output;

converting said analog digital output into a digital output;

and

recording said digital output on digital magnetic tape.

2. The method of claim 1 further including the steps of timemultiplexingand recordin tar et position information with the recorded dlgital dataw ere y t e variation of said range profiles with respect to targetaspect variation can be computed by said general purpose digitalcomputers.

3. The method of recording on digital magnetic tape highrange resolutionradar target signatures comprising the steps of:

illuminating a remote target with an RF pulse having a range resolutionof approximately 1 foot; detecting received target signatures; samplingin a moving sample-point mode a selectively predetermined number ofsuccessive detected target signatures to thereby produce an analogsampled output;

converting said analog sampled output into a digital output;

and

recording said digital output on digital magnetic tape.

1. The method of converting analog high-range resolution radar data fromremote targets into digital data compatible with general purpose digitalcomputers comprising the steps of: illuminating a target underinvestigation with linear FM pulses of microwave energy having a rangeresolution of approximately 1 foot; receiving successive reflectedone-dimensional range profiles of microwave energy backscatter along thelength of said targets; said range profiles being unique for eachdifferent remote target in accordance with the geometric configurationof the scatter centers on each of said different remote target;detecting each of said successive reflected range profiles from saidtarget under investigation; sampling each of the detected successiverange profiles at a selectively predetermined rate and at incrementallyincreasing range positions corresponding to the target length to therebyproduce an analog digital output; converting said analog digital outputinto a digital output; and recording said digital output on digitalmagnetic tape.
 2. The method of claim 1 further including the steps oftime-multiplexing and recording target position information with therecorded digital data whereby the variation of said range profiles withrespect to target aspect variation can be computed by said generalpurpose digital computers.
 3. The method of recording on digitalmagnetic tape high-range resolution radar target signatures comprisingthe steps of: illuminating a remote target with an RF pulse having arange resolution of approximately 1 foot; detecting received targetsignatures; sampling in a moving sample-point mode a selectivelypredetermined number of successive detected target signatures to therebyproduce an analog sampled output; converting said analog sampled outputinto a digital output; and recording said digital output on digitalmagnetic tape.